While this invention relates broadly to facility monitoring and control systems in which data collected at remote locations is transmitted to a central control unit over a cable, the subject invention will first be described in connection with multi-wire intrusion detection systems in which the data to be transmitted back to the control unit is of a relatively simple nature. In terms of alarm signals indicating intrusion, the signal sent back to the control unit may merely be the presence of a pulse to indicate that an alarm condition has occurred. Ordinarily there is no further information needed because the sensors at the remote locations are all of a similar type; that is they detect only one type of alarm condition.
As will be discussed hereinafter, the subject invention also encompasses transmission of more complex data which may include discrimination between different types of alarms such as fire or intrusion and in a further embodiment, the monitoring of room temperatures, boiler pressure, elevator operation, emergency lighting, etc.
However, in its least complicated aspect, the subject zone indication system is first described in terms of the annunciation of an intrusion.
In multi-wire intrusion detection systems, in which numbers of sensors are connected in parallel to a multi-wire cable, when an intrusion or tampering is detected, it is often times difficult to ascertain the location of the intrusion or the tampering site. Thus, if a guard or other responsible personnel is to respond to an alarm indication, it is difficult to know where the breach of security occurred and therefore it is difficult to know how to respond.
Annunciator systems in the past have required a separate cable or set of cables in order to indicate the side of the alarm condition. Running multiple wires in addition to those already utilized for the intrusion detection system is both costly and sometimes impossible depending on the installation.
Audible alarms at the site of the intrusion or security breach while giving an audible indication of the breach location are often times ineffective in that they give the would be intruder sufficient warning to escape. Moreover, these systems measures to prevent capture which may endanger the security personnel investigating the breach. Additionally, audible alarms at the sensor location are ineffective to alert security personnel to the loction of the intrusion if the central control unit is not within bearing of all of the individual transceivers or sensors.
In order to provide zone indication with an in-place multi-wire cable, a digital coding system might be utilized in which each of the transceivers connected to the multi-wire cable would be given a different address code which could be polled by the central unit on a time shared basis, with the central unit generating a series of codes which when detected by a particular receiver unit would result in the coupling of the transceiver unit output to the multi-wire cable system. This of course would require considerable code recognition circuitry, and code generating circuitry. Training for the installers of such systems would additionally be complicated by knowledge of the code and code setting procedures. Moreover, some means would be necessary to separate out the zone code signals from the command signals normally utilized to control the individual sensors connected to the multi-cable line. Command signals are ordinarily digital in nature and are thus little different from the digital signals which would be sent in order to poll a given sensor.
It might be thought that a better type of zone indication system which would utilize the existing cables would be one in which polling was accomplished acoustically or at least at acoustic frequencies, with each sensor being assigned a predetermined audio frequency. Tuning fork frequency determining elements could conceivably be utilized in the sensor such that the sensors could be polled by an acoustic signal of the appropriate frequency.
However, it will be appreciated that the generation of multiple differing acoustic frequencies is both expensive and electronically somewhat complicated, as is the filtering system necessary to filter out the responses from the sensor in terms of the acoustic subcarrier which would be utilized. Moreover, since ultrasonic equipment is sometimes utilized at the sensor to detect intrusion, crosstalk could occur between the ultrasonic devices and the acoustic devices in the annunciator system.
In the digitally encoded systems described above, or in the acoustically encoded system described, the false alarm rate is unduly high in view of the aforementioned crosstalk between sensor/command signals and zone indication signals transmitted on the same lines. If separate lines are utilized for zone indication, system costs greatly increase if such a system were in fact feasible for a given location.